Where traditional methods of bonding the roof and shell of Floating Roof Tanks fall short, and a new method excels

Petroleum storage tank fires due to lightning or static discharge are more common than most people think. In fact, dozens of oil industry tank fires are reported each year in the worldwide media, ranging from rim seal fires to multiple, simultaneous full tank fires. Many of these costly incidents are attributed to lightning or static discharge.

A key strategy to preventing these types of petroleum tank fires is to properly bond the roof and wall of the industry's Floating Roof Tanks (FRTs). This can prevent lightning or static discharge-related electrical arcing between the two, a frequent trigger of these costly fires. Unfortunately, traditional methods of bonding the roof and wall of FRTs face serious drawbacks including: high impedance at lightning frequencies; high likelihood of arcing at seals; difficult inspection; a sub-optimal number of bonds; and impaired performance caused by condition of the tank wall.

Recently, companies as varied as ExxonMobil, Shell, Petroleos de Venezuela (PDVSA), B.P., ChevronTexaco, and Bahamas Oil Refining have been adopting a new grounding method developed to address these concerns. The new grounding method, known as Retractable Grounding Assembly (RGA), is providing increased protection from lightning and static discharge-related FRT fires, while minimizing the need for traditional grounding maintenance.

Imperfect Seals and Flammable Vapor

FRTs, in which the roof floats on top of the product stored, are commonly used to store petroleum products such as crude oil, gasoline, and diesel fuel. To prevent vapors from escaping from around the edge of the roof, flexible seals are fitted around the roof's edge. Since the seal material is non-conductive, it electrically isolates the roof from the tank shell and any connection to earth.

Unfortunately, these seals are not perfect. The seals become worn, cracked and/or damaged over time. In addition, the tank shell often becomes warped and out-of-round due to repeated filling, draining, heating, cooling, etc. The tank shell's inner surface can also become uneven from corrosion and/or petroleum residue, such as tar and paraffin. Because of these imperfections, extremely combustible petroleum vapor can leak from around the seals, which is why the region above the roof in a FRT is classified as a Class I Division I hazardous area.

How Lightning and Static Discharge Cause Tank Fires

A lightning strike can deliver up to 10 billion volts and 510,000 amps. This can occur within about 13 milliseconds, with temperatures reaching up to 50,000 F. Even an average lightning strike delivers about 30,000 amps of electricity to ground within a few milliseconds. These tremendous currents flow from the thundercloud to the earth each time the accumulated charges exceed the insulating thresholds of the atmosphere.

The problem: if lightning terminates on or near a FRT, large currents can flow across the roof-shell interface. If the impedance between the roof and shell is high, electrical arcing or sparking will occur across the seal interface, which can ignite a fire. This fire hazard can also be triggered by static discharge inducing activities such as filling a tank, when petroleum product may pick up an electrical charge passing through pipes.

Not only can a direct lightning strike to the tank create arcing, but also strikes to any nearby structure - such as other tanks, buildings, trees, poles, fences, or the ground - can do so. Even strikes to "protective" equipment such as lightning rods or ESEs can cause dangerous arcing.

To prevent the fire hazard of electrical arcing, the floating roof must be electrically bonded to the tank shell in order to equalize electrical potential. This has traditionally been accomplished with shunts, a walkway, or a bonding cable, though these methods have significant drawbacks.

The Limitations of Traditional Roof -Shell Bonding Solutions Shunts

To create the roof-shell bond, FRT owners usually install metal finger-like devices called "shunts." Shunts are attached to the roof and must be in constant contact with the tank shell. NFPA 780 requires shunts to be spaced no more than every ten feet around the tank roof perimeter, but the contact resistance of shunts depends on their composition, contact pressure, and the condition of the tank shell.

Unfortunately, shunts do not provide a certain, low resistance bond to the tank shell for several reasons, including:

1. Heavy crude oil components, such as tar and paraffin tend to coat the inside of the tank shell, forming an insulative barrier between the shell and shunts.

2. Any corrosion (rust) on the inside of the shell creates a high resistance connection between the shell and shunts.

3. If the inside of the tank is painted, the paint will insulate the shell from the shunts.

4. Large tanks are typically out-of-round by several inches. In these cases, shunts tend to pull away from the tank shell, eliminating the grounding connection that prevents the electrical arcing fire hazard.

Moreover, independent third-party testing, performed in cooperation with the API and the Energy Institute in England, has shown that arcing will occur at the shunt-shell interface under all lightning conditions, whether or not the shunts are above the roof or submerged. If the shunts are above the roof, however, arcing occurs in the worst possible place: in a Class I Division I hazardous area, which may have a highly ignitable concentration of fuel-air vapor.

Walkway Ladder

Another way to create the roof-shell bond is to rely on a walkway ladder from the top of the tank shell to the roof. Nearly all FRT's have a walkway ladder with the upper end attached to the tank rim and the lower end riding on roof-mounted rails. As the floating roof rises and falls, the ladder's lower end rolls to compensate for the roof's changing height.

However, the quality of the electrical connection via the ladder is questionable. Since the ladder's upper end is a bolted hinge, it is subject to poor electrical conductivity due to looseness, corrosion and insulative paint. Similarly, the electrical connection on the ladder's lower end, a pressure connection through two wheels riding on rails, is subject to high-impedance caused by corrosion and insulative paint.

Because of these uncertain connections, large potential differences will occur between the roof and shell using a walkway ladder, and arcing can occur at the seal locations during lightning activity or instances of severe static build up.

Roof-Shell Bonding Cable

A third way to create a roof-shell bond is with a bonding cable between the top of the tank shell and the middle of its roof. The cable is typically bonded to the shell's rim, suspended along the bottom of a ladder, and bonded to the roof's center. The cable must be long enough to accommodate the roof in its lowest position. Because of the relatively high impedance of this cable under lightning conditions, plus the fact that there is only one such cable per tank, this type of bond is considered a high impedance connection.

For instance, for a 200 foot diameter tank 50 feet high, the cable must be at least 112 feet long to reach the roof's center when the tank is empty. Though at 60Hz this cable has low impedance, at lightning frequencies it has high impedance. At a frequency of 100 kHz, for example, the impedance of 100 feet of typical 0.5" diameter or 250 MCM roof-shell bonding cable is over 15 ohms. This is too high to prevent arcing at the shunts during lightning events, when thousands of electrical amps may flow across the tank.

Thus, with a single roof-shell bonding cable, large potential differences will still occur between the roof and shell along with hazardous electrical arcing at the seal locations.

The Benefits of New Retractable Cable Roof-Shell Bonding

The ideal bond between the FRT roof and shell would have low impedance across a wide range of frequencies; be easy to install on new tanks and retrofit onto existing tanks; and be easy to inspect, test, and replace if necessary.

To satisfy these requirements, Lightning Eliminators & Consultants, Inc. (LEC) of Boulder, Colorado developed the Retractable Grounding Assembly (RGA). The RGA provides a very low impedance, direct connection between the tank roof and shell, using a wide thick-braided wire cable, spring-loaded on a heavy stainless steel reel. It is easy to install on new and existing tanks, as well as easy to inspect, test, and maintain.

With impedance of one ohm or less - compared to shunts or walkway ladders with impedance as high as 500 ohms - the RGA offers a reliable, full-time grounding connection that can help prevent lightning or static discharge-related petroleum fires.

The RGA's path of impedance is kept to a practical minimum by a combination of the shortest path, wide braid, and constant spring tension. Its spring-loaded reel extends cable as the roof descends, and retracts it as the roof rises - so the line remains taut at the minimal distance needed for grounding, assuring minimal impedance and faster, more reliable grounding.

Unlike traditional roof-shell bonding methods, the RGA's wide braid maximizes surface area and therefore conductivity, since high frequency electrical charges (electrons) actually travel most effectively along the surfaces of wire conductors. The RGA cable, constructed from 864 strands of #30 AWG copper wire, is braided together to form a strap 1.625" wide by 0.11" thick, and is tinned for extra corrosion protection. With over twice the surface area of typical 0.5" diameter or 250 MCM roof-shell bonding cable, for instance, the RGA offers significantly better conductivity.

Since the RGA functions independently of the condition of the tank shell, it can eliminate the need for more traditional roof-shell bonding methods like shunts, or serve as a more effective primary safety system for preventing lightning or static discharge-related fire hazard.

Unlike a walkway ladder or single roof-shell bonding cable, multiple RGAs can be used to ensure multiple positive bonds between the tank shell and roof, and the lowest likelihood of arcing at a seal.

Because of the numerous advantages of RGAs, they are being used worldwide by companies such as ExxonMobil, ChevronTexaco, Shell, B.P., Petroleos de Venezuela (PDVSA), and Bahamas Oil Refining, often alongside other lightning protection equipment such as LEC's Dissipation Array System (DASŪ).

"We chose RGA to improve the protection of the tank from bound charges," said Greg Cooper, an Engineering Manager at Bahamas Oil Refining, whose company installed RGAs on 15 crude oil and 4 product tanks with floating roofs.

"For four years prior to 2000 we experienced five lightning strikes which resulted in rim fires with many more before that," says Cooper, whose company previously relied on shunt plates, along with cables attached to tank shells and roof ladders.

"We installed the Dissipation Array System and RGAs during 2000," says Cooper. "I feel that the Dissipation Array System and the RGAs definitely work."

Lightning Eliminators & Consultants, Inc. designs, manufactures, and installs integrated, engineered systems such as the RGA and DAS to prevent and eliminate grounding and other lightning related problems.